https://support.apple.com/en-us/HT211683 in 2021 in January. Im uploading this because of archive reasons and older Macs that can't run newer versions of Mac OS. This is mainly for archival and unsupported systems. If you get a certificate error or validation go into terminal when booted into the installer and do enter this command sudo date 0201010121 this command will set your date to February first 2021. After doing this in the terminal in the Mac OS installer, reboot the computer and try again. If the date sets automatically disconnect from the internet, and rerun the command again then restart.

For more information look here 

https://tidbits.com/2019/10/28/redownload-archived-macos-installers-to-address-expired-certificates/